1. Field of the Invention
This invention relates to a method of biologically removing nitrogen and a system therefor, and more particularly to a system for biologically removing nitrogen, in which waste water containing ammonium nitrogen at high concentration is nitrified and denitrified by use of fixed microorganisms, whereby nitrogen contained in the waste water is removed.
2. Description of the Related Art
Since ammonium nitrogen contained in the waste water becomes one of the materials causing decrease of dissolved oxygen in a water area for public use, into which effluent flows in and also causing eutrophy in a closed water area, it is necessary to remove ammonium nitrogen in the waste water.
As a method of biologically removing ammonium nitrogen in the waste water, normally, there is a process by use of a single sludge predenitrification process, in which there are utilized nitrifying reaction from ammonia to nitric acid by use of nitrifying bacteria and denitrifying reaction from nitric acid to nitrogen by use of denitrifying bacteria. According to this method, two tanks including a denitrifying tank in the anaerobic condition and a nitrifying tank in the aerobic condition are used, in the nitrifying tank, decomposition of the organic substance and the denitrifying process are performed by the denitrifying bacteria, and, in the nitrifying tank, ammonium nitrogen in the waste water is nitrified into nitric acid by the nitrifying bacteria. Then, a nitrified liquid which is nitrified in the nitrifying tank is recycled through the denitrifying tank, whereby a nitrogen component in the waste water is discharged into condition as nitrogen gas and removed. In this single sludge predenitrification process, in order to improve the nitrifying efficiency, fixing of the nitrifying bacteria is examined, and a process, in which the fixed nitrifying bacteria are thrown into the nitrifying tank is used in practice. Further, a process, in which the fixed denitrifying bacteria are thrown into the denitrifying tank, is examined.
The waste water, to which this single sludge predenitrification process is applied, is chiefly sewerage having the ammonium nitrogen concentration (NH.sub.4 --N) as low as about 20.about.60 mg/l, and, in the ammonium nitrogen concentration as low as this extent, nitrifying reaction proceeds smoothly, whereby the ammonium nitrogen concentration in the effluent is decreased to several mg/l less than the effluent standard in the river area.
However, in the case where the waste water containing the high ammonium nitrogen concentration (approximately, 400 mg/l.about.5000 mg/l) is produced in great quantity such for example as in a developing laboratory, an inorganic matter synthesizing factory, a power plant and the like, the ammonium nitrogen concentration of the original waste water is diluted to less than 200 mg/l, and then, the biological process is performed. As the result, such a problem occurs that the amount of the waste water to be processed is increased considerably, whereby a large-sized system for biologically removing nitrogen is needed, however, it is difficult to install the large-sized system in the cities where any large installation area cannot be obtained.
As a method of biologically processing the ammonium nitrogen waste water having the high concentration, the inventors of the invention of the present application have previously developed a method of processing at high speed by a multiple stage process by use of microorganisms entrapped in organic gel. According to this method, the waste water is caused to flow in series through three nitrifying tanks (aeration tanks), into which media of microorganisms entrapped in organic gel are thrown. For example, in the case where operating conditions are selected such that the ammonium nitrogen concentration of the waste water (the ammonium nitrogen concentration of the waste water, which was caused to flow into a first thank) was 250 mg/l, the amount of the processed waste water was 480 m.sup.3 /day and the residence time was 7 hours, the quality of the processed water in a third tank was decreased to a value as low as 37.5 mg/l. Furthermore, since the nitrifying speed of media in the first tank has shown a very high value of 123 mg-N/h.multidot.l-medium, this fact was highly noted as one which made the system to be compact in size. This nitrifying speed is an important factor highly influencing on the rate of removal of nitrogen for determining a required capacity of an aeration thank. The higher the nitrifying speed is, the more the system can be made compact in size.
As described above, necessity has been voiced for making the system for biologically removing nitrogen to be compact in size. In particular, when the system for biologically removing nitrogen for processing the ammonium nitrogen waste water having the high concentration can be made to be compact in size, the usefulness is very high.
However, in the above-described case of multiple stage process, when the waste water, in which the ammonium nitrogen concentration is as high as several thousand mg/l, there is such a problem that the dilution magnification is still required to be increased, and it is still unsatisfactory for making the system to be compact in size.
Because of this, necessity has been voiced for the development of a system for biologically removing nitrogen capable of highly processing, in which the ammonium nitrogen waste water having the high concentration can be processed as it is not diluted or at a concentration of a low dilution magnification.
Now, as for the nitrifying and denitrifying processes, such a concept is generalized that in the nitrifying process, the ammonium nitrogen is oxidized into nitrous acid by bacteria which are generally called Nitrosomonas, then, oxidized into nitric acid by Nitrobactor, and subsequently, reduced into nitrogen gas in the denitrifying process. However, if, at the stage of nitrous acid which is an intermediate oxide material in the nitrifying process, nitrous acid is moved to denitrifying process, i.e., nitrifying and denitrifying reactions of nitrous acid type can be carried out, then, the reaction process can be shortened and the processing time can be decreased according, so that this fact contributes to making the system compact in size, and moreover, it is expected to raise the reacting speed.